1. The Field of the Invention
The present invention is directed to polymeric materials used to form protective coatings on electrochromic devices. The present invention is also directed to a process for preparing electrochromic devices having a protective polymeric coating thereon.
2. The Relevant Technology
Electrochromic devices are optical devices typically composed of thin layers of inorganic oxides that change color in response to an applied voltage. Electrochromic devices have many useful applications, such as intentionally darkening windows, mirrors, eyeglasses, filters and electro-optical devices. A problem with electrochromic devices, however, is that electrochromic devices lack the long term durability to be practically useful.
Electrochromic devices lack the structural and chemical durability to have any viable commercial application. Even the slightest physical contact can damage an electrochromic structure and render the electrochromic device unusable. Furthermore, any scratch or other blemish on the surface of the electrochromic device can adversely effect the optical qualities of the device. In short, electrochromic devices are exposed to physical contact that destroys, or significantly shortens the lifetime of electrochromic devices.
Similarly, electrochromic devices have a delicate chemical make up that is easily altered by exposure to reactive substances, such as moisture, oxygen, cleaning solutions and other contaminants. Any variation in the chemical nature of the electrochromic layers, whether from the evolution of necessary elements from the device, or the invasion of reactive elements into the device, adversely effects, or destroys, the proper functioning of the electrochromic device. Hence, for the electrochromic device to be economically viable, it is vital that any physical or chemical contact with the electrochromic device be greatly reduced or completely avoided.
In an attempt to make electrochromic devices commercially viable, many people have tried to increase the mechanical and environmental durability of electrochromic devices by coating the electrochromic devices with polymers, reaction curing-type resins, adhesive resins and other various materials. While many of these coating materials have been effective barriers to environmental or mechanical elements, none of the proposed coatings have provided complete protection from both environmental and mechanical damage. For instance, many polymeric materials form effective environmental or chemical barriers, but are much less effective in protecting electrochromic devices against mechanical impact, such as scratching, handling, or other contact (Japanese Application No. 58-91431(A); Baucke et al. U.S. Pat. No. 4,465,339; Ganguillet et al., U.S. Pat. No. 4,392,720; Bissar et al., U.S. Pat. No. 4,227,779; and Amano, U.S. Pat. No. 4,403,831). Similarly, coating materials that form effective mechanical barriers are commonly porous and, thus, do not provide adequate environmental or chemical protection for electrochromic devices.
To provide an environmental and mechanical barrier for electrochromic cells, Agrawal U.S. Pat. No. 4,852,979, discloses the use of two layers of protective materials, one layer immediately adjacent to the electrochromic cell that protects the cell from mechanical damage and an outer layer covering the entire area of the electrochromic cell, including the mechanical barrier, that protects the cell from reactive elements, such as moisture and oxygen. The combination of these two layers provides a coating that effectively protects the electrochromic device from mechanical and electrical damage.
Unfortunately, the use of two layers is time consuming and expensive. Furthermore, as mentioned above, electrochromic devices are optical devices and any coating added to the electrochromic device must not adversely effect the optical characteristics of the device. Therefore, the coatings must have a specific refractive index, optical emissivity, thermal expansion coefficient, etc. The greater the number of layers added to the electrochromic device, the greater the possibility that the optical qualities will be affected. A further drawback of the process in Agrawal is that the materials used to form the protective layers cannot be exposed to temperatures greater than 65.degree. C. during processing. This severely limits the curing process and coating materials that may be used in coating formation.
From the foregoing, it is readily apparent that there is a need for a coating material that can be used to protect electrochromic devices from environmental as well as mechanical damage while maintaining the optical qualities of the electrochromic device. Furthermore, it is clear that there is a need for an efficient process for coating electrochromic devices.